The present invention relates to mold assemblies and methods for molding plastic articles and to the molded articles produced thereby. More particularly, the invention relates to an improved injection mold assembly configured to produce an integrally molded plastic tub having recessed drain holes and to a method for producing such a tub.
A basket or tub of the type used in clothes washing machines includes a circular bottom and cylindricall sidewalls extending generally perpendicularly from the bottom. It is known to cast such tubs of metal and to coat the tubs with porcelain to provide a smooth exterior and clean appearance. These tubs also include a plurality of drain holes that can be formed during the casting operation.
Washing machine tubs are also made of molded plastic. While plastic offers weight and cost advantages over metal, plastic tubs formed by conventional methods require assembly or further finishing of the molded product to produce a completed tub. In particular, in some prior art tub making techniques the drain holes must be drilled into the molded tub and deburred and in some cases it is also necessary to countersink or otherwise recess the holes. These finishing operations are time consuming and labor intensive.
To avoid the abovementioned assembling and finishing operations, injection mold assemblies configured to produce molded tubs have in the past employed metal core pins which are engageable with an opposing mold surface to form the drain holes in the tub. However, known injection mold assemblies have proven expensive to maintain and susceptable to wear. In particular, the core pins must each be precisely machined within tight tolerences to insure that each core pin is the correct length to match properly with the opposing mold surface. Repeated engagement between the core pins and the opposing mold surfaces results in wear on both the core pins and the mold surfaces, thereby permitting plastic to enter the space between the core pins and the opposed mold surfaces to form flashing over the drain holes. This flashing must be removed in a subsequent manufacturing step and thus increases the cost of the tub. Also, worn core pins must be replaced by core pins machined to the exact length as the core pins they are replacing and this further increases the cost of producing injection molded plastic tubs.
Additionally, the removal of molded tubs from injection molding machines has proven difficult due to the shape of the tub and to the provision of circular raised mold surfaces intended to provide for the recessing of the drain holes. These raised mold surfaces hinder the ability of the tub to slide off a central mold core and to be removed from the molding machine.